Method for starting an internal combustion engine of a hybrid drive of a motor vehicle

ABSTRACT

A method for starting an internal combustion engine of a hybrid drive of a motor vehicle, the internal combustion engine and at least one electric motor being mechanically linkable to an output shaft of the hybrid drive via a gear system. The output shaft of the hybrid drive is placed in rotary motion, and at least one of the electric motors is switched to a generator mode by the gear system and by a control system.

FIELD OF THE INVENTION

The present invention relates to a method of starting an internalcombustion engine of a hybrid drive of a motor vehicle, the internalcombustion engine and at least one electric motor being mechanicallylinkable to an output shaft of the hybrid drive via a gear system.

BACKGROUND INFORMATION

Hybrid drives are known. Hybrid drives have multiple drive sources thatmay be operated by a range of different energy sources.

For example, methods of combining an internal combustion engine with atleast one electric motor are known. In this case, the crankshaft of theinternal combustion engine and the drive shaft of the at least oneelectric motor are connected as input shafts to a gear system (shiftmechanism). The internal combustion engine and/or the electric motor aremechanically linkable to an output shaft of the hybrid drive byoperating the gear system. The output shaft is used in the known mannerto drive a motor vehicle. In particular, hybrid systems that includemore than one electric motor are also known.

In this case, in particular, motor vehicles having power branchinghybrid drives are known, which, in addition to an internal combustionengine, include two electric motors that may be both motor-driven andgenerator-driven. Motor vehicles having hybrid drives of this type donot need to include a separate starting system for the internalcombustion engine. The internal combustion engine may be started by theat least one electric motor of the hybrid drive by connecting the latterto an energy source, usually the motor vehicle battery. When the atleast one electric motor is operating in motor mode, the internalcombustion engine is started by shifting the gear system accordingly.

The disadvantage of the known hybrid drives is that the internalcombustion engine does not start if the power supply for the at leastone electric motor has failed.

SUMMARY OF THE INVENTION

The method according to the present invention has the advantage over therelated art that the internal combustion engine may be started even ifthe supply voltage for the at least one electric motor of the hybriddrive has failed. By switching the at least one electric motor togenerator mode, using the gear system of the hybrid drive, and placingthe output shaft of the gear system in rotary motion, it isadvantageously possible, even in the case of hybrid drives, to carry outauxiliary starting of the hybrid drive by applying an external kineticenergy to the motor vehicle. The external kinetic energy may be appliedto the motor vehicle, for example by pushing, towing or rolling on aninclined plane or through similar means. Switching the at least oneelectric motor to generator mode establishes a mechanical link betweenthe output shaft of the gear system and the crankshaft of the internalcombustion engine. This makes it possible to transmit the kinetic energyfrom the output shaft to the crankshaft of the internal combustionengine, thus starting the latter.

According to a preferred embodiment of the present invention, one of theelectric motors is switched to generator mode in the case of a hybriddrive having two electric motors, and the electrical energy supplied bythis electric motor running in generator mode is used to operate the atleast one further electric motor in motor mode. This enables theelectric motor operating in motor mode to help start the internalcombustion engine by applying an external kinetic energy to the motorvehicle, once the electric motor operating in generator mode hasprovided sufficient electrical energy.

According to a further preferred embodiment of the present invention,the at least one electric motor, in particular all the electric motors,of the hybrid system are first operated in idle mode and the electricmotors are switched to generator mode or motor mode only after apresettable amount of external kinetic energy has been applied to themotor vehicle. For example, a rotational speed of the output shaft maybe used as a criterion for this purpose. This makes it easier to apply asufficient amount of kinetic energy to the motor vehicle, since it isnot necessary to first overcome a resistance of the electric motors.

According to a further preferred embodiment of the present invention,the hybrid drive is started by first operating the electric motor inidle mode and, after applying a presettable amount of external kineticenergy to the motor vehicle, braking the electric motor throughmechanical, pneumatic, hydraulic or other suitable means. Doing sodirectly mechanically couples the output shaft of the gear system to thecrankshaft of the internal combustion engine, allowing the internalcombustion engine to start.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a hybrid drive of a motor vehicle.

FIG. 2 shows a block diagram of different starting variants of theinternal combustion engine of the hybrid drive.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of a motor vehicle hybrid drivethat is identified as a whole by reference number 10. Hybrid drive 10includes an internal combustion engine 12 as well as a first electricmotor 14 and a second electric motor 16. A crankshaft 18 of internalcombustion engine 12 and drive shafts 20 and 22, respectively, ofelectric motors 14 and 16 are mechanically linked to a gear system 24.In this case, drive shaft 20 is connected to a first planetary gear 26and drive shaft 22 is connected to a second planetary gear 28. A ringgear of planetary gear 26 is connected to a shift mechanism 30, and aring gear of planetary gear 28 is connected to a shift mechanism 32.Shift mechanisms 30 and 32, in turn, are mechanically linked to anoutput shaft 34 of gear system 24. Output shaft 34 is mechanicallylinked to a driving axle of the motor vehicle, which is not illustrated.

Because the design and function of a hybrid drive 10 of this type aregenerally known, they are not discussed in greater detail within thescope of the present description. In particular, internal combustionengine 12 and/or electric motors 14 and 16 are selectively actuatable totransmit a variable input torque from these components to output shaft34. This makes it possible to set different operating modes of hybriddrive 10. By operating a switch in the known manner, shift mechanisms 30and 32 allow different gears to be engaged, which are identified here asgears 1, 2, 3, 4, 5 and 6 as well as reverse gear R. Electric motors 14and 16 may each be operated in generator mode and are used, for example,to provide a vehicle electrical system voltage of the motor vehicle andto charge a battery. To electric motors 14 and 16 are assigned brakeunits 36 and 38, respectively, which enable rotors of electric motors 14and 16 to be mechanically braked.

During normal operation, internal combustion engine 12 of hybrid drive10 is started in the known manner by operating at least one of electricmotors 14 and 16 in motor mode. For this purpose, electric motor 14and/or 16 is connected to a vehicle electrical system supply, usually amotor vehicle battery.

In the explanation below, it is assumed that it is not possible to startinternal combustion engine 12 by operating it in a mode that is normalper se. This occurs, for example, when the motor vehicle batteryresponsible for the electric motors is flat or defective. In thissituation, the electrical energy in the motor vehicle would not beavailable for starting internal combustion engine 12.

Below is an explanation of three embodiments according to which internalcombustion engine 12 may be started without using the availableelectrical energy source. In all three embodiments, output shaft 34 ofhybrid drive 10 is placed in rotary motion to start internal combustionengine 12. This may be done, for example, by pushing or towing the motorvehicle. By mechanically linking output shaft 34 to a drive system(drive wheels) of the motor vehicle, the motor vehicle motion isconverted to a rotary motion of output shaft 34 when a kinetic energy isapplied to the motor vehicle. It is also possible to have the motorvehicle roll on an inclined plane, for example a downhill road orsimilar surface.

According to the first embodiment, electric motors 14 and 16 are firstswitched to idle mode, i.e., they are not in motor mode or generatormode. Shift mechanisms 30 and 32 are placed in defined positions. Thismay be done, for example, by shifting shift mechanism 30 to reverse gearR and shift mechanism 32 to second gear 2. For this purpose, thecountershafts of shift mechanisms 30 and 32 are placed in theappropriate positions. After drive shaft 34 is placed in rotary motion,electric motors 14 and 16 rotate in different directions, depending onthe coupling between drive shafts 20 and 22 and gear system 24. Electricmotor 16 is placed in generator mode, while electric motor 14 is placedin motor mode. Output shaft 34 is thus mechanically coupled tocrankshaft 18 of internal combustion engine 12 via gear system 24 sothat crankshaft 18 rotates simultaneously with the rotation of outputshaft 34 and serves to start internal combustion engine 12 in a mannerthat is known per se. Because electric motor 16 is in generator mode, itsupplies an electrical energy to the motor vehicle electrical system.Electric motor 14 may additionally use this electrical energy for motormode so that the resulting input torque present at electric motor 14 mayalso be used to start internal combustion engine 12.

According to a second embodiment, electric motors 14 and 16 are firstoperated in idle mode. Defined gears are engaged in shift mechanisms 30and 32. For example, the countershafts are used to engage shiftmechanism 30 in first gear 1 and shift mechanism 32 in second gear 2.After a kinetic energy has been applied to the motor vehicle, i.e.,while the latter is in motion and output shaft 34 is rotating, electricmotors 14 and 16 are each placed in generator mode. Both electric motors14 and 16 thus transmit a rectified torque to crankshaft 18 via gearsystem 24 so that the rotation of crankshaft 18 may be used to startinternal combustion engine 12. By placing electric motors 14 and 16 ingenerator mode, it is possible to simultaneously supply electricalenergy to the vehicle battery. As a result, electrical energy issimultaneously available once again for the vehicle electrical supply—inparticular, if it is not possible to start the vehicle normally becausethe battery is empty.

According to a third embodiment, electric motors 14 and 16 are firstswitched to idle mode while output shaft 34 is rotating. Defined gearswere engaged beforehand. After output shaft 34 has been placed in rotarymotion, at least one of electric motors 14 or 16 is braked by activatingbrake unit 36 or 38. This couples output shaft 34 to crankshaft 18directly via gear system 24. This direct coupling now enables internalcombustion engine 12 to start.

The discussions above make it clear that different strategies may beemployed to start internal combustion engine 12, the requirement beingthat output shaft 34 is always rotated by applying a kinetic energy tothe motor vehicle. The decision as to which of the possible strategiesis used in the end may be determined by a higher-level engine controlunit of hybrid drive 10.

FIG. 2 shows a block diagram illustrating the different ways in whichthe starting strategies for internal combustion engine 12 may beprocessed. After initiating a start command 40, a check is first carriedout in step 42 to determine whether electric motors 14 and 16 areoperational. If electric motors 14 and 16 are operational, a check iscarried out in a subsequent step 44 to determine whether full actuationof electric motors 14 and 16 is possible. If so, internal combustionengine 12 may be started according to method variant 46 described first.This means that electric motor 14 is switched to generator mode andelectric motor 16 to motor mode. Subsequently, starting 48 of internalcombustion engine 12 takes place.

If query 44 reveals that, while electric motors 14 and 16 areoperational, full actuation of electric motors 14 and 16 is not ensured,a check is carried out in a step 50 to determine whether electric motors14 and 16 may be run at least in generator mode, if not in motor mode.If this is possible, internal combustion engine 12 is started 48′ in astep 52 according to the second embodiment explained.

If query 42 reveals that electric motors 14 and 16 are not operational,and if query 50 shows that electric motors 14 and 16 may not be operatedin generator mode, a check is carried out in a step 54 to determinewhether brake units 36 and 38 of electric motors 14 and 16 areoperational. If they are, third variant 48″ for starting internalcombustion engine 12 is initiated in a step 56. If query 54 reveals thatbrake units 36 and 38 are not operational, a decision 58 is made that itis not possible to start internal combustion engine 12 by pushing,towing or rolling on an inclined plane.

Variants 46, 52 and 56 for starting internal combustion engine 12 alsobuild upon each other redundantly. If preceding queries 42 and 44 revealthat variant 46 for starting internal combustion engine 12 is possibleper se, but internal combustion engine 12 does not start, the method mayautomatically go on to variant 52 and, if necessary, automatically tovariant 56. If variant 56 is also unable to start internal combustionengine 12, decision 58 is made that it is not possible to start internalcombustion engine 12.

On the whole, it is clear that, due to the branched decision hierarchy,internal combustion engine 12 of hybrid drive 10 may be easilyemergency-started by applying a kinetic energy to the motor vehicle, forexample if the vehicle electrical system voltage supply fails.

The present invention is, of course, not limited to the illustratedexemplary embodiment of a hybrid drive 10. Other types of hybrid drives10, for example having more or fewer than two electric motors or using adifferent type of geared coupling between internal combustion engine 12and electric motors 14 and 16, are also possible.

1. A method for starting an internal combustion engine of a hybrid driveof a standing motor vehicle in case of a failure of an electric powersupply, the internal combustion engine and at least one electric motorbeing mechanically linkable to an output shaft of the hybrid drive via agear system, the method comprising: placing the output shaft of thehybrid drive in rotary motion; switching at least one of the at leastone electric motor to a generator mode by the gear system and by acontrol system, wherein each of the at least one electric motor of thehybrid drive is first operated in idle mode, and the at least oneelectric motor is switched to generator mode only after a presettableamount of external kinetic energy has been applied to the motor vehicleby one of pushing the vehicle, towing the vehicle, and pushing thevehicle down an inclined plane; and transmitting the kinetic energy fromthe output shaft to a crankshaft of the internal combustion engine,thereby cranking the internal combustion engine.
 2. The method accordingto claim 1, further comprising using electrical energy supplied by theelectric motor operating in generator mode to operate at least onesecond electric motor in motor mode.
 3. The method according to claim 1,further comprising selecting one of generator mode and motor mode of theelectric motor by operating a shift mechanism of the gear system.
 4. Amethod for starting an internal combustion engine of a hybrid drive of astanding motor vehicle in case of a failure of electric power supply,the internal combustion engine and at least one electric motor beingmechanically linkable to an output shaft of the hybrid drive via a gearsystem, the method comprising: to start the internal combustion engine,first operating the at least one electric motor in idle mode and placingthe output shaft in rotary motion by one of pushing the vehicle, towingthe vehicle, and pushing the vehicle down an inclined plane;subsequently braking the at least one electric motor in at least one ofa mechanical, pneumatic and hydraulic manner; and transmitting thekinetic energy from the output shaft to a crankshaft of the internalcombustion engine, thereby cranking the internal combustion engine. 5.The method according to claim 4, wherein the output shaft is placed inrotary motion by applying a kinetic energy to the motor vehicle.
 6. Themethod according to claim 4, further comprising selecting one of aplurality of available method variants for starting the internalcombustion engine by a control unit of the hybrid drive.